Simulation plays an important role in the development, testing and evaluation of new robotic applications, reducing implementation time, cost and risk. For much of the robotics community, the open-source Gazebo robot simulator has emerged as the de facto standard environment for prototyping and testing robotic systems. While Gazebo offers strong support for terrestrial, aerial and space robotics applications, less support is available for marine applications involving vehicles at and below the water surface. To address this deficiency, we present the Virtual RobotX (VRX) simulation, a general purpose open-source development and testing tool, based on Gazebo, capable of approximating the behavior of unmanned surface vessels operating in complex ocean environments. We highlight the application of these capabilities using the VRX challenge reference implementation, a new simulation-based robot competition designed to complement the physical Maritime RobotX Challenge.